A major U.S. hospital operator says hackers based in China broke into its computer systems and stole data on 4.5 million patients.

Community Health Systems said the attack occurred in April and June of this year, but it wasn’t until July that it determined the theft had taken place.

Working with a computer security company, it determined the attack was carried out by a group based in China that used “highly sophisticated malware” to attack its systems. It didn’t release specific details of the attack.

The group is apparently known to U.S. federal law enforcement authorities, which are now involved in the case. But the identity of the group was not disclosed.

The hackers got away with patient names, addresses, birthdates, telephone numbers and Social Security numbers of the 4.5 million people who were referred to or received services from doctors affiliated with the company in the last five years. The stolen data did not include patient credit card, medical or clinical information.

Community Health Systems, based in Franklin, Tennessee, is one of the largest hospital operators in the U.S. with 206 hospitals in 29 states from New York across the South to the Pacific coast.

The attack ranks as the second largest disclosed attack to hit the U.S. medical industry in the last few years, according to data from the Department of Health and Human Services.

The only one that was bigger involved data on 4.9 million military clinic and hospital patients stolen from Science Applications International Disclosed in 2011, the loss happened when data backup tapes containing records from 1992 through September 2011 were lost.

A second incident that comes close was the theft of a personal computer from Advocate Medical Group in a 2013 burglary that contained information on around 4 million patients.